This invention relates to an improved power supply or feeder for an ion pump.
On the basis of the technical knowledge about the ion pumps, the pumping speed or rate for a given pressure should be proportional to the ion current and therefore to the voltage applied across the electrodes; as a consequence, the pumping speed should increase with the voltage. While such phoenomenon has been verified in the pressure range from 10.sup.-7 to 10.sup.-5 mbar, at pressures lower than 10.sup.-7 mbar the pumping speed of an ion pump does not appear to show any longer a behaviour proportional to the voltage applied to its electrodes.
Thus the problem arises to determine the voltage values adapted to maximise the pump performances in the different pressure ranges at which it may be operated.
U.S. Pat. No. 3,429,501 to Hamilton et al. relates to an ion pump fed by a first voltage at low pressures--and therefore at low currents--which is higher than the voltage supplied to the pump at higher pressures, in order to keep constant at the optimum value the supplied power.
U.S. Pat. No. 4,713,619 of the same Applicant relates to a feeder for an ion pump wherein a suitable electronic circuit alternatively switches between two feeding voltages--a high one and a low one--independentely of the current. The two voltage cyclic feeding aims to reduce the influence of the field effect current on the overall current and to allow the use of the ion pump as a pressure measuring device even of very low pressures (below 10.sup.-6 mbar) thanks to an extension of the linear range of the current/pressure characteristic.
Nevertheless neither of the above mentioned patents provides for a solution to optimize the pump performances at low pressures, nor considers the influence of the feeding voltage on the pumping speed.
Therefore the present invention aims to eliminate or to reduce the inconveniences of the known feeding systems for ion pumps, by providing a feeder which is able to optimize the pump performances in every pressure range, particularly at the lower pressures (below 10.sup.-7 mbar) which further allows for the use of the pump as a pressure measuring device.